The present invention relates to a copying apparatus utilizing a slit exposure system in which the image of a flat stationary original is focused onto a photosensitive member through a movable mirror performing a parallel displacement and a path-length compensating optical system.
As an exposure scanning system with a stationary original support table there is already known, as disclosed in the U.S. Pat. No. 3,330,181, a scanning system wherein the optical path length during the scanning is maintained constant by means of a first mirror displacing at a constant speed synchronized with the displacement of a photosensitive member, and of a second mirror performing a parallel displacement at a speed equal to a half of that of said first mirror while maintaining a predetermined positional relationship to said first mirror. However, this system is defective in requiring a complicated actuating mechanism, as said first and second mirrors have to be maintained in said mutual relationship during the scanning, and also in the possible undesirable effect of the mirror vibration on the image resolution on the photosensitive member, as the system involves two movable mirrors. On the other hand, in the U.S. Pat. No. 3,537,373 there is disclosed a scanning system in which a stationary original support table is scanned by a rotating planar mirror, and the change in the optical path length is compensated by means of an imaging lens. In this system, though it is rendered possible to simplify the actuating mechanism and to remove the undesirable effect of vibration because of the absence of displacing mirrors for scanning, an exact image cannot be obtained due to image distortion resulting from the scanning with a rotating mirror.